/// <summary>
 /// 發送郵件
 /// </summary>
 public void sendmail ()
 {
  // 創建tcpclient對象， 并建立連接
  tcpclient tcp = null;
  try
  {
  tcp = new tcpclient (server, port);
  }
  catch (exception)
  {
  throw new exception ("無法連接服務器");
  }

  readstring (tcp.getstream());//獲取連接信息

  // 開始進行服務器認證
  // 如果狀態碼是250則表示操作成功
  if (!command (tcp.getstream(), "ehlo localhost", "250"))
  throw new exception ("登陸階段失敗");

  if (username != "")
  {
  // 需要身份驗證
  if (!command (tcp.getstream(), "auth login", "334"))
   throw new exception ("身份驗證階段失敗");
  string nameb64 = tobase64 (username); // 此處將username轉換為base64碼
  if (!command (tcp.getstream(), nameb64, "334"))
   throw new exception ("身份驗證階段失敗");
  string passb64 = tobase64 (password); // 此處將password轉換為base64碼
  if (!command (tcp.getstream(), passb64, "235"))
   throw new exception ("身份驗證階段失敗");
  }

  // 準備發送
  writestring (tcp.getstream(), "mail from: " + from);
  writestring (tcp.getstream(), "rcpt to: " + to);
  writestring (tcp.getstream(), "data");

  // 發送郵件頭
  writestring (tcp.getstream(), "date: " + datetime.now); // 時間
  writestring (tcp.getstream(), "from: " + fromname + "<" + from + ">"); // 發件人
  writestring (tcp.getstream(), "subject: " + subject); // 主題
  writestring (tcp.getstream(), "to:" + toname + "<" + to + ">"); // 收件人

  //郵件格式
  writestring (tcp.getstream(), "content-type: multipart/mixed; boundary="unique-boundary-1"");
  writestring (tcp.getstream(), "reply-to:" + from); // 回復地址
  writestring (tcp.getstream(), "x-priority:" + priority); // 優先級
  writestring (tcp.getstream(), "mime-version:1.0"); // mime版本

  // 數據id,隨意
//  writestring (tcp.getstream(), "message-id: " + datetime.now.tofiletime() + "@security.com");
  writestring (tcp.getstream(), "content-transfer-encoding:" + encoding); // 內容編碼
  writestring (tcp.getstream(), "x-mailer:jcpersonal.utility.mailsender"); // 郵件發送者
  writestring (tcp.getstream(), "");

  writestring (tcp.getstream(), tobase64 ("this is a multi-part message in mime format."));
  writestring (tcp.getstream(), "");

  // 從此處開始進行分隔輸入
  writestring (tcp.getstream(), "--unique-boundary-1");

  // 在此處定義第二個分隔符
  writestring (tcp.getstream(), "content-type: multipart/alternative;boundary="unique-boundary-2"");
  writestring (tcp.getstream(), "");

  if(!ishtml)
  {
  // 文本信息
  writestring (tcp.getstream(), "--unique-boundary-2");
  writestring (tcp.getstream(), "content-type: text/plain;charset=" + languageencoding);
  writestring (tcp.getstream(), "content-transfer-encoding:" + encoding);
  writestring (tcp.getstream(), "");
  writestring (tcp.getstream(), body);
  writestring (tcp.getstream(), "");//一個部分寫完之后就寫如空信息，分段
  writestring (tcp.getstream(), "--unique-boundary-2--");//分隔符的結束符號，尾巴后面多了--
  writestring (tcp.getstream(), "");
  }
  else
  {
  //html信息
  writestring (tcp.getstream(), "--unique-boundary-2");
  writestring (tcp.getstream(), "content-type: text/html;charset=" + languageencoding);
  writestring (tcp.getstream(), "content-transfer-encoding:" + encoding);
  writestring (tcp.getstream(), "");
  writestring (tcp.getstream(), htmlbody);
  writestring (tcp.getstream(), "");
  writestring (tcp.getstream(), "--unique-boundary-2--");//分隔符的結束符號，尾巴后面多了--
  writestring (tcp.getstream(), "");
  }

  // 發送附件
  // 對文件列表做循環
  for (int i = 0; i < attachments.count; i++)
  {
  writestring (tcp.getstream(), "--unique-boundary-1"); // 郵件內容分隔符
  writestring (tcp.getstream(), "content-type: application/octet-stream;name="" + ((attachmentinfo)attachments).filename + """); // 文件格式
  writestring (tcp.getstream(), "content-transfer-encoding: base64"); // 內容的編碼
  writestring (tcp.getstream(), "content-disposition:attachment;filename="" + ((attachmentinfo)attachments).filename + """); // 文件名
  writestring (tcp.getstream(), "");
  writestring (tcp.getstream(), ((attachmentinfo)attachments).bytes); // 寫入文件的內容
  writestring (tcp.getstream(), "");
  }

  command (tcp.getstream(), ".", "250"); // 最后寫完了，輸入"."

  // 關閉連接
  tcp.close ();
 }

 /// <summary>
 /// 向流中寫入字符
 /// </summary>
 /// <param name="netstream">來自tcpclient的流</param>
 /// <param name="str">寫入的字符</param>
 protected void writestring (networkstream netstream, string str)
 {
  str = str + " "; // 加入換行符

  // 將命令行轉化為byte[]
  byte[] bwrite = encoding.getencoding(languageencoding).getbytes(str.tochararray());

  // 由于每次寫入的數據大小是有限制的，那么我們將每次寫入的數據長度定在７５個字節，一旦命令長度超過了７５，就分步寫入。
  int start=0;
  int length=bwrite.length;
  int page=0;
  int size=75;
  int count=size;
  try
  {
  if (length>75)
  {
   // 數據分頁
   if ((length/size)*size<length)
   page=length/size+1;
   else
   page=length/size;
   for (int i=0;i<page;i++)
   {
   start=i*size;
   if (i==page-1)
    count=length-(i*size);
   netstream.write(bwrite,start,count);// 將數據寫入到服務器上
   }
  }
  else
   netstream.write(bwrite,0,bwrite.length);
  }
  catch(exception)
  {
  // 忽略錯誤
  }
 }

 /// <summary>
 /// 從流中讀取字符
 /// </summary>
 /// <param name="netstream">來自tcpclient的流</param>
 /// <returns>讀取的字符</returns>
 protected string readstring (networkstream netstream)
 {
  string sp = null;
  byte[] by = new byte[1024];
  int size = netstream.read(by,0,by.length);// 讀取數據流
  if (size > 0)
  {
  sp = encoding.default.getstring(by);// 轉化為string
  }
  return sp;
 }

 /// <summary>
 /// 發出命令并判斷返回信息是否正確
 /// </summary>
 /// <param name="netstream">來自tcpclient的流</param>
 /// <param name="command">命令</param>
 /// <param name="state">正確的狀態碼</param>
 /// <returns>是否正確</returns>
 protected bool command (networkstream netstream, string command, string state)
 {
  string sp=null;
  bool success=false;
  try
  {
  writestring (netstream, command);// 寫入命令
  sp = readstring (netstream);// 接受返回信息
  if (sp.indexof(state) != -1)// 判斷狀態碼是否正確
   success=true;
  }
  catch(exception)
  {
  // 忽略錯誤
  }
  return success;
 }

 /// <summary>
 /// 字符串編碼為base64
 /// </summary>
 /// <param name="str">字符串</param>
 /// <returns>base64編碼的字符串</returns>
 protected string tobase64 (string str)
 {
  try
  {
  byte[] by = encoding.default.getbytes (str.tochararray());
  str = convert.tobase64string (by);
  }
  catch(exception)
  {
  // 忽略錯誤
  }
  return str;
 }

 /// <summary>
 /// 附件信息
 /// </summary>
 public struct attachmentinfo
 {
  /// <summary>
  /// 附件的文件名 [如果輸入路徑，則自動轉換為文件名]
  /// </summary>
  public string filename {
  get { return filename; }
  set { filename = path.getfilename(value); }
  } private string filename;

  /// <summary>
  /// 附件的內容 [由經base64編碼的字節組成]
  /// </summary>
  public string bytes {
  get { return bytes; }
  set { if (value != bytes) bytes = value; }
  } private string bytes;

  /// <summary>
  /// 從流中讀取附件內容并構造
  /// </summary>
  /// <param name="ifilename">附件的文件名</param>
  /// <param name="stream">流</param>
  public attachmentinfo (string ifilename, stream stream)
  {
  filename = path.getfilename (ifilename);
  byte[] by = new byte [stream.length];
  stream.read (by,0,(int)stream.length); // 讀取文件內容
  //格式轉換
  bytes = convert.tobase64string (by); // 轉化為base64編碼
  }

  /// <summary>
  /// 按照給定的字節構造附件
  /// </summary>
  /// <param name="ifilename">附件的文件名</param>
  /// <param name="ibytes">附件的內容 [字節]</param>
  public attachmentinfo (string ifilename, byte[] ibytes)
  {
  filename = path.getfilename (ifilename);
  bytes = convert.tobase64string (ibytes); // 轉化為base64編碼
  }

  /// <summary>
  /// 從文件載入并構造
  /// </summary>
  /// <param name="path"></param>
  public attachmentinfo (string path)
  {
  filename = path.getfilename (path);
  filestream file = new filestream (path, filemode.open);
  byte[] by = new byte [file.length];
  file.read (by,0,(int)file.length); // 讀取文件內容
  //格式轉換
  bytes = convert.tobase64string (by); // 轉化為base64編碼
  file.close ();
  }
 }
}
}

--------------------------------------------------------------------------------

// 使用:

 mailsender ms = new mailsender ();
 ms.from = "[email protected]";
 ms.to = "[email protected]";
 ms.subject = "subject";
 ms.body = "body text";
 ms.username = "########"; // 怎么能告訴你呢
 ms.password = "********"; // 怎么能告訴你呢
 ms.server = "smtp.tom.com";

 ms.attachments.add (new mailsender.attachmentinfo (@"d: est.txt"));

 console.writeline ("mail sending...");
 try
 {
  ms.sendmail ();
  console.writeline ("mail sended.");
 }
 catch (exception e)
 {
  console.writeline (e);
 }